1. Field of the Invention
The present invention relates to a heat exchanging apparatus such as absorption refrigerating machines and heat pumps having a corrosion prevention function added, and a method of preventing corrosion therein.
2. Description of the Background Art
A heat exchanging apparatus such as an absorption refrigerating machine and a heat pump is well known. An absorption refrigerating machine will be described hereinafter as an example of a heat exchanging apparatus. The absorption refrigerating machine includes a household refrigerator. Refrigeration by this absorption refrigerating machine is carried out utilizing the cooling effect resulting from the evaporation of a liquid refrigerant. Although various kinds are available as a refrigerant, an embodiment of an absorption refrigerating machine using ammonia as a refrigerant and water as an absorbent will be described. This absorption refrigerating machine includes a generator, a rectifier, a condenser, an evaporator, and an absorber, whereby a refrigeration cycle is implemented.
Concentrated ammonia solution introduced into the refrigeration cycle is heated in the generator to be separated into vapor and ammonia gas. The vapor is then condensed in a rectifier, whereas ammonia gas is condensed in a condenser. This ammonia gas is fed to the evaporator as ammonia liquid. The vapor condensed in the rectifier is supplied to an absorber as dilute ammonia solution. In the evaporator, ammonia liquid is evaporated to extract latent heat of vaporization from its periphery, resulting in a refrigeration effect. Ammonia liquid after being evaporated is conducted to the absorber to be mixed with dilute ammonia solution. As a result, concentrated ammonia solution is produced in the absorber, which is conducted to the generator. This embodiment of a refrigeration cycle will be repeated.
The concentration of the concentrated ammonia solution fed to the generator is typically 30-35%. The heating temperature of the generator is approximately 170.degree. C. The pressure in the generator is 15-20 Kg/cm.sup.2. The pressure reaches 30 Kg/cm.sup.2 in diffusion type generators. Carbon steel is typically known as the material that can be used under the above-described conditions. Carbon steel is said to be corrosion-resistant to ammonium and ammonia solution. However, in practice, it will corrode in places which achieve high temperature and high pressure such as in a generator. If corrosion-preventing measures are not taken such as adding an appropriate corrosion inhibitor in the ammonia solution, the generator will be corroded and damaged on account of its high pressure interior.
A strong oxidant such as chromate which is an inorganic based oxidant is often used as a corrosion inhibitor. Chromate is added into an ammonia solution and sealed within the refrigeration cycle. On operating the absorption refrigerating machine, a thin layer is formed on the internal surface of the generator by oxidation of chromate. This thin layer serves to prevent corrosion.
Although the above-described chromate is superior regarding corrosion-resistant performance, Cr.sup.6+ has a disadvantage of industrial pollution because of its strong toxicity.